The 70-kilodalton heat-shock proteins of the SSA subfamily negatively modulate heat-shock-induced accumulation of trehalose and promote recovery from heat stress in the yeast, Saccharomyces cerevisiae

Metatranscriptomics Reveals Sequential Expression of Genes Involved in the Production of Melanogenesis Inhibitors by the Defined Microbial Species in Fermented Unpolished Black Rice

Fermented products require metabolic enzymes from the microbial community for desired final products. Using a metatranscriptomic approach, the role of microorganisms in fermented products on producing compounds with a melanogenesis inhibition activity has not yet been reported. Previously, unpolished black rice (UBR) fermented with the E11 starter containing Saccharomyces cerevisiae, Saccharomycopsis fibuligera, Rhizopus oryzae, and Pediococcus pentosaceus (FUBR) showed potent melanogenesis inhibition activity. This study aimed to investigate the function of these defined microbial species in producing melanogenesis inhibitors in the FUBR using a metatranscriptomic approach. The melanogenesis inhibition activity increased in a fermentation time-dependent manner. Genes related to melanogenesis inhibitors synthesis such as carbohydrate metabolism, amino acids synthesis, fatty acids/unsaturated fatty acids synthesis, and carbohydrate transporters were analyzed. Most genes from R. oryzae and P. pentosaceus were upregulated in the early stage of the fermentation process, while those of S. cerevisiae and S. fibuligera were upregulated in the late stage. FUBR production using different combinations of the four microbial species shows that all species were required to produce the highest activity. The FUBR containing at least R. oryzae and/or P. pentosaceus exhibited a certain level of activity. These findings were in agreement with the metatranscriptomic results. Overall, the results suggested that all four species sequentially and/or coordinately synthesized metabolites during the fermentation that led to a FUBR with maximum melanogenesis inhibition activity. This study not only sheds light on crucial functions of certain microbial community on producing the melanogenesis inhibitors, but also paves the way to initiate quality improvement of melanogenesis inhibition activity in the FUBR. IMPORTANCE Fermentation of food is a metabolic process through the action of enzymes from certain microorganisms. Although roles of the microbial community in the fermented food were investigated using metatranscriptomic approach in terms of flavors, but no study has been reported so far on the function of the microorganisms on producing compounds with a melanogenesis inhibition activity. Therefore, this study explained the roles of the defined microorganisms from the selected starter in the fermented unpolished black rice (FUBR) that can produce melanogenesis inhibitor(s) using metatranscriptomic analysis. Genes from different species were upregulated at different fermentation time. All four microbial species in the FUBR sequentially and/or coordinately synthesized metabolites during fermentation that led to a FUBR with maximal melanogenesis inhibition activity. This finding contributes to a deeper understanding of the roles of certain microbial community during fermentation and led to the knowledge-based improvement for the fermented rice with potent melanogenesis inhibition activity.

Transient intracellular acidification regulates the core transcriptional heat shock response

Heat shock induces a conserved transcriptional program regulated by heat shock factor 1 (Hsf1) in eukaryotic cells. Activation of this heat shock response is triggered by heat-induced misfolding of newly synthesized polypeptides, and so has been thought to depend on ongoing protein synthesis. Here, using the budding yeast Saccharomyces cerevisiae, we report the discovery that Hsf1 can be robustly activated when protein synthesis is inhibited, so long as cells undergo cytosolic acidification. Heat shock has long been known to cause transient intracellular acidification which, for reasons which have remained unclear, is associated with increased stress resistance in eukaryotes. We demonstrate that acidification is required for heat shock response induction in translationally inhibited cells, and specifically affects Hsf1 activation. Physiological heat-triggered acidification also increases population fitness and promotes cell cycle reentry following heat shock. Our results uncover a previously unknown adaptive dimension of the well-studied eukaryotic heat shock response.

The same but different: the role of Hsp70 in heat shock response and prion propagation

The Hsp70 chaperone machinery is a key component of the heat-shock response and a modulator of prion propagation in yeast. A major factor in optimizing Hsp70 function is the highly coordinated activities of the nucleotide-binding and substrate-binding domains of the protein. Hsp70 inter-domain communication occurs through a bidirectional allosteric interaction network between the two domains. Recent findings identified the β6/β7 region of the substrate-binding domain as playing a critical role in optimizing Hsp70 function in both the stress response and prion propagation and highlighted the allosteric interaction interface between the domains. Importantly, while functional changes in Hsp70 can result in phenotypic consequences for both the stress response and prion propagation, there can be significant differences in the levels of phenotypic impact that such changes illicit.

Freezing tolerance of sea urchin embryonic cells: Differentiation commitment and cytoskeletal disturbances in culture

This study focuses on the freezing tolerance of sea urchin embryonic cells. To significantly reduce the loss of physiological activity of these cells that occurs after cryopreservation and to study the effects of ultra-low temperatures on sea urchin embryonic cells, we tested the ability of the cells to differentiate into spiculogenic or pigment directions in culture, including an evaluation of the expression of some genes involved in pigment differentiation. A morphological analysis of cytoskeletal disturbances after freezing in a combination of penetrating (dimethyl sulfoxide and ethylene glycol) and non-penetrating (trehalose and polyvinylpyrrolidone) cryoprotectants revealed that the distribution pattern of filamentous actin and tubulin was similar to that in the control cultures. In contrast, very rare spreading cells and a small number of cells with filamentous actin and tubulin were detected after freezing in the presence of only non-penetrating cryoprotectants. The largest number of pigment cells was found in cultures frozen with trehalose or trehalose and dimethyl sulfoxide. The ability to induce the spicule formation was lost in the cells frozen only with non-penetrating cryoprotectants, while it was maximal in cultures frozen in a cryoprotective mixture containing both non-penetrating and penetrating cryoprotectants (particularly, when ethylene glycol was present). Using different markers for cell state assessment, an effective cryopreservation protocol for sea urchin cells was developed: three-step freezing with a low cooling rate (1-2°C/min) and a combination of non-penetrating and penetrating cryoprotectants made it possible to obtain a high level of cell viability (up to 65-80%).

Acquisition of thermotolerant yeast Saccharomyces cerevisiae by breeding via stepwise adaptation

A thermotolerant Saccharomyces cerevisiae yeast strain, YK60-1, was bred from a parental strain, MT8-1, via stepwise adaptation. YK60-1 grew at 40°C, a temperature at which MT8-1 could not grow at all. YK60-1 exhibited faster growth than MT8-1 at 30°C. To investigate the mechanisms how MT8-1 acquired thermotolerance, DNA microarray analysis was performed. The analysis revealed the induction of stress-responsive genes such as those encoding heat shock proteins and trehalose biosynthetic enzymes in YK60-1. Furthermore, nontargeting metabolome analysis showed that YK60-1 accumulated more trehalose, a metabolite that contributes to stress tolerance in yeast, than MT8-1. In conclusion, S. cerevisiae MT8-1 acquired thermotolerance by induction of specific stress-responsive genes and enhanced intracellular trehalose levels.

Local and systemic mycorrhiza-induced protection against the ectoparasitic nematode Xiphinema index involves priming of defence gene responses in grapevine

The ectoparasitic dagger nematode (Xiphinema index), vector of Grapevine fanleaf virus (GFLV), provokes gall formation and can cause severe damage to the root system of grapevines. Mycorrhiza formation by Glomus (syn. Rhizophagus) intraradices BEG141 reduced both gall formation on roots of the grapevine rootstock SO4 (Vitis berlandieri×V. riparia) and nematode number in the surrounding soil. Suppressive effects increased with time and were greater when the nematode was post-inoculated rather than co-inoculated with the arbuscular mycorrhizal (AM) fungus. Using a split-root system, decreased X. index development was shown in mycorrhizal and non-mycorrhizal parts of mycorrhizal root systems, indicating that both local and systemic induced bioprotection mechanisms were active against the ectoparasitic nematode. Expression analyses of ESTs (expressed sequence tags) generated in an SSH (subtractive suppressive hybridization) library, representing plant genes up-regulated during mycorrhiza-induced control of X. index, and of described grapevine defence genes showed activation of chitinase 1b, pathogenesis-related 10, glutathione S-transferase, stilbene synthase 1, 5-enolpyruvyl shikimate-3-phosphate synthase, and a heat shock proein 70-interacting protein in association with the observed local and/or systemic induced bioprotection against the nematode. Overall, the data suggest priming of grapevine defence responses by the AM fungus and transmission of a plant-mediated signal to non-mycorrhizal tissues. Grapevine gene responses during AM-induced local and systemic bioprotection against X. index point to biological processes that are related either to direct effects on the nematode or to protection against nematode-imposed stress to maintain root tissue integrity.

Influence of heat shock on glycerol production in alcohol fermentation

The influence of single and double heat shocks induced during the exponential growth phase of the Saccharomyces cerevisiae fermentation of cultivar Sauvignon Blanc grape must was examined. Rapid temperature changes from 18 degrees C to 34 degrees C have been applied. The effect of the duration of exposure to a high temperature has been analyzed. By the applications of a single heat shock and a double heat shock, up to 8.2 g l(-1) and 11.0 g l(-1) glycerol have been produced, respectively. To prevent the evaporation of fine wine bouquet compounds during the temperature changes, reflux coolers on the top of bioreactors have been employed. By using this method, glycerol production was increased by up to 65%.

Role of trehalose and heat in the structure of the C-terminal activation domain of the heat shock transcription factor

The heat shock transcription factor (HSF) is the primary transcriptional regulator of the heat shock response in eukaryotes. Saccharomyces cerevisiae HSF1 has two functional transcriptional activation domains, located N- and C-terminal to the central core of the protein. These activation domains have a low level of transcriptional activity prior to stress, but they acquire a high level of transcriptional activity in response to stresses such as heat. Previous studies on the N-terminal activation domain have shown that it can be completely disordered. In contrast, we show that the C-terminal activation domain of S. cerevisiae HSF1 does contain a certain amount of secondary structure as measured by circular dichroism (CD) and protease resistance. The alpha-helical content of the domain can be increased by the addition of the disaccharide trehalose but not by sucrose. Trehalose, but not sucrose, causes a blue shift in the fluorescence emission spectra, which is suggestive of an increase in tertiary structure. Trehalose, which is known to be a chemical chaperone, also increases proteases' resistance and promotes heat-induced increases in alpha-helicity. The latter is particularly intriguing because of the physiological role of trehalose in yeast. Trehalose levels are increased dramatically after heat shock, and this is thought to protect protein structure prior to the increase of heat shock protein levels. Our results suggest that the dramatic changes in S. cerevisiae HSF1 transcriptional activity in response to stress might be linked to the combined effects of trehalose and elevated temperatures in modifying the overall structure of HSF1's C-terminal activation domain.

Both heat-shock and cold-shock influence trehalose metabolism in an entomopathogenic nematode

Heat-shock response is highly conserved in animals and microorganisms, and it results in the synthesis of heat-shock proteins. In yeast, heat-shock response has also been reported to induce trehalose accumulation. We explored the relationship between heat- (35 C) or cold-shock (1 and 10 C) and trehalose metabolism in the entomopathogenic nematode, Heterorhabditis bacteriophora. Because both heat- and cold-shocks may precede desiccation stress in natural soil environments, we hypothesized that nematodes may accumulate a general desiccation protectant, trehalose, under both situations. Indeed, both heat- and cold-shocks influenced trehalose accumulation and activities of enzymes of trehalose metabolism in H. bacteriophora. Trehalose increased by 5- and 6-fold in heat- and cold-shocked infective juveniles, respectively, within 3 hr of exposure, compared with the nematodes maintained at 25 C (culture temperature). The activity of trehalose-6-phosphate synthase (T6PS), an enzyme involved in the synthesis of trehalose, also significantly increased in both heat- and cold-shocked nematodes during the first 3 hr of exposure. Generally, the trehalose levels and activities of T6PS declined to their original levels within 3 hr when nematodes were transferred back to 25 C. In both heat- and cold-shocked nematodes, trehalase activity decreased significantly within the first 3 hr and generally returned to the original levels within 3 hr when these nematodes were transferred back to 25 C. The results demonstrate that the trehalose concentrations in H. bacteriophora are influenced by both heat- and cold-shocks and are regulated by the action of 2 trehalose-metabolizing enzymes, T6PS and trehalase. The accumulated trehalose may enhance survival of nematodes under both cold and warm conditions, but it may also provide simultaneous protection against desiccation that may result from subsequent evaporation or freezing. This is the first report of the relationship between trehalose metabolism and heat-shock for the Nematoda.

The trehalose myth revisited: introduction to a symposium on stabilization of cells in the dry state

This essay is an introduction to a series of papers arising from a symposium on stabilization of cells in the dry state. Nearly all of these investigations have utilized the sugar trehalose as a stabilizing molecule. Over the past two decades a myth has grown up about special properties of trehalose for stabilization of biomaterials. We review many of such uses here and show that under ideal conditions for drying and storage trehalose has few, if any, special properties. However, under suboptimal conditions trehalose has some distinct advantages and thus may remain the preferred excipient. We review the available mechanisms for introducing trehalose into the cytoplasm of living cells as an introduction to the papers that follow.

Role for de novo sphingoid base biosynthesis in the heat-induced transient cell cycle arrest of Saccharomyces cerevisiae

The recent findings of sphingolipids as potential mediators of yeast heat stress responses led us to investigate their possible role in the heat-induced cell cycle arrest and subsequent recovery. The sphingolipid-deficient yeast strain 7R4 was found to lack the cell cycle arrest seen in the isogenic wild type. Furthermore, strain lcb1-100, which harbors a temperature-sensitive serine palmitoyltransferase, lacked increased de novo generated sphingoid bases upon heat stress. Importantly, this strain was found to lack the transient heat-induced G0/G1 arrest. These results indicate a role for sphingolipids and specifically those generated in the de novo pathway in the cell cycle arrest response to heat. To determine the bioactive sphingolipid regulating this response, an analysis of key mutants in the sphingolipid biosynthetic and degradation pathways was performed. Strains deleted in sphingoid base kinases, sphingoid phosphate phosphatase, lyase, or dihydrosphingosine hydroxylase were found to display the cell cycle arrest. Also, the knockout of a fatty acyl elongation enzyme, which severely attenuates ceramide production, displayed the arrest. These experiments suggested that the active species for cell cycle arrest were the sphingoid bases. In further support of these findings, exogenous phytosphingosine (10 microM) was found to induce transient arrest. Stearylamine did not induce an arrest, demonstrating chemical specificity, and L-erythro- was not as potent as D-erythro-dihydrosphingosine showing stereospecificity. To investigate a possible arrest mechanism, we studied the hyperstable Cln3 (Cln3-1) strain LDW6A that has been previously shown to be resistant to heat stress-induced cell cycle arrest. The strain containing Cln3-1 was found to be resistant to cell cycle arrest induced by exogenous phytosphingosine, indicating that Cln3 acts downstream of the sphingoid bases in this response. Interestingly, cell cycle recovery from the transient arrest was found to be dependent upon the sphingoid base kinases (LCB4, LCB5). Overall, this combination of genetic and pharmacologic results demonstrates a role for de novo sphingoid base biosynthesis by serine palmitoyltransferase in the transient G0/G1 arrest mediated through Cln3 via a novel mechanism.

Anticancer drugs as inducers of thermotolerance in yeast

Yeast cell viability was evaluated microscopically following exposure to heat shock for 30 min at 53 degrees C. The cells were previously grown in the presence of potential stressors (anticancer drugs; e.g., 5-fluorouracil, methotrexate, cisplatin, bleomycin, mitomycin-C and camptothecin-11). The induction of thermotolerance was documented by significantly increased viability after heat shock. This effect, which was reversed by cycloheximide, was comparable to that observed following exposure to a mild heat stress. These data demonstrate that pretreatment with sub-toxic concentrations of some of the clinically used antineoplastic agents conferres thermotolerance to yeast, possibly through the synthesis of protein components.

Stress tolerance in a yeast lipid mutant: membrane lipids influence tolerance to heat and ethanol independently of heat shock proteins and trehalose

The response of a yeast unsaturated fatty acid auxotroph, defective in delta 9-desaturase activity, to heat and ethanol stresses was examined. The most heat- and ethanol-tolerant cells had membranes enriched with oleic acid (C18:1), followed in order by cells enriched with linoleic (C18:2) and linolenic (C18:3) acids. Cells subjected to a heat shock (25-37 degrees C for 30 min) accumulated trehalose and synthesized typical heat shock proteins. Although there were no obvious differences in protein profiles attributable to lipid supplementation of the mutant, relative protein synthesis as determined by densitometric analysis of autoradiograms suggested that hsp expression was different. However, there was no consistent relationship between the synthesis of heat shock proteins and the acquisition of thermotolerance in the lipid supplemented auxotroph or related wild type. Furthermore, trehalose accumulation was also not closely related to stress tolerance. On the other hand, the data presented indicated a more consistent role for membrane lipid composition in stress tolerance than trehalose, heat shock proteins, or ergosterol. We suggest that the sensitivity of C18:3-enriched cells to heat and ethanol may be attributable to membrane damage associated with increases in membrane fluidity and oxygen-derived free radical attack of membrane lipids.

Antagonistic interactions between yeast chaperones Hsp104 and Hsp70 in prion curing

The maintenance of [PSI], a prion-like form of the yeast release factor Sup35, requires a specific concentration of the chaperone protein Hsp104: either deletion or overexpression of Hsp104 will cure cells of [PSI]. A major puzzle of these studies was that overexpression of Hsp104 alone, from a heterologous promoter, cures cells of [PSI] very efficiently, yet the natural induction of Hsp104 with heat shock, stationary-phase growth, or sporulation does not. These observations pointed to a mechanism for protecting the genetic information carried by the [PSI] element from vicissitudes of the environment. Here, we show that simultaneous overexpression of Ssa1, a protein of the Hsp70 family, protects [PSI] from curing by overexpression of Hsp104. Ssa1 protein belongs to the Ssa subfamily, members of which are normally induced with Hsp104 during heat shock, stationary-phase growth, and sporulation. At the molecular level, excess Ssa1 prevents a shift of Sup35 protein from the insoluble (prion) to the soluble (cellular) state in the presence of excess Hsp104. Overexpression of Ssa1 also increases nonsense suppression by [PSI] when Hsp104 is expressed at its normal level. In contrast, hsp104 deletion strains lose [PSI] even in the presence of overproduced Ssa1. Overproduction of the unrelated chaperone protein Hsp82 (Hsp90) neither cured [PSI] nor antagonized the [PSI]-curing effect of overproduced Hsp104. Our results suggest it is the interplay between Hsp104 and Hsp70 that allows the maintenance of [PSI] under natural growth conditions.

Ssa1p chaperone interacts with the guanine nucleotide exchange factor of ras Cdc25p and controls the cAMP pathway in Saccharomyces cerevisiae

We have found that the guanine nucleotide exchange factor for ras, Cdc25p, interacts with Ssa1p in Saccharomyces cerevisiae. This interaction was observed with GST-fused Cdc25p polypeptides and confirmed by coimmunoprecipitation with the endogenous Cdc25p. Hsp82 appeared also to be co-immunoprecipitated with Cdc25p, albeit to a lower level than Hsp70. In a strain deleted for SSA1 and SSA2, we observed a reduced cellular content of Cdc25p. Consistent with a reduced activity of the cAMP-dependent PKA pathway, the rate of accumulation of both trehalose and glycogen was stimulated in the ssa-deleted strain. Expression of SSA1 reversed these effects, whereas co-expression of SSA1 and PDE2 restored high accumulation. The expression of genes repressed by cAMP, GAC1 and TPS1, fused to beta-galactosidase, was also stimulated by deletion of SSA genes. The effect of ssa deletion on glycogen accumulation was lost in a strain deleted for CDC25 rescued by the RAS2ile152 allele. Altogether, these results lead to the conclusion that Ssa1p positively controls the cAMP pathway through Cdc25p. We propose that this connection plays a critical role in the adaptation of cells to stress conditions.

Evidence for the interplay between trehalose metabolism and Hsp104 in yeast

Disruption of the HSP104 gene in a mutant which cannot accumulate trehalose during heat shock treatment caused trehalose accumulation (H. Iwahashi, K. Obuchi, S. Fujii, and Y. Komatsu, Lett. Appl. Microbiol 25:43-47, 1997). This implies that Hsp104 affects trehalose metabolism. Thus, we measured the activities of enzymes involved in trehalose metabolism. The activities of trehalose-synthesizing and -hydrolyzing enzymes are low in the HSP104 disruption mutant during heat shock. This data is correlated with intracellular trehalose and glucose levels observed in the HSP104 disruption mutant. These results suggest that during heat shock, Hsp104 contributes to the simultaneous increase in both accumulation and degradation of trehalose.

Biotechnological applications of the disaccharide trehalose

Trehalose is a disaccharide present in a variety of anhydrobiotic organisms which have the ability to promptly resume their metabolism after addition of water. It has been successfully used as a nontoxic cryoprotectant of enzymes, membranes, vaccines, animal and plant cells and organs for surgical transplants. It has been predicted that trehalose can also be used as an ingredient for dried and processed food. Therefore, the recent biotechnological applications of trehalose have imposed the standardization of methods for its production, as well as for its specific quantification.

Proteasome inhibitors cause induction of heat shock proteins and trehalose, which together confer thermotolerance in Saccharomyces cerevisiae

An accumulation in cells of unfolded proteins is believed to be the common signal triggering the induction of heat shock proteins (hsps). Accordingly, in Saccharomyces cerevisiae, inhibition of protein breakdown at 30 degrees C with the proteasome inhibitor MG132 caused a coordinate induction of many heat shock proteins within 1 to 2 h. Concomitantly, MG132, at concentrations that had little or no effect on growth rate, caused a dramatic increase in the cells' resistance to very high temperature. The magnitude of this effect depended on the extent and duration of the inhibition of proteolysis. A similar induction of hsps and thermotolerance was seen with another proteasome inhibitor, clasto-lactacystin beta-lactone, but not with an inhibitor of vacuolar proteases. Surprisingly, when the reversible inhibitor MG132 was removed, thermotolerance decreased rapidly, while synthesis of hsps continued to increase. In addition, exposure to MG132 and 37 degrees C together had synergistic effects in promoting thermotolerance but did not increase hsp expression beyond that seen with either stimulus alone. Although thermotolerance did not correlate with hsp content, another thermoprotectant trehalose accumulated upon exposure of cells to MG132, and the cellular content of this disaccharide, unlike that of hsps, quickly decreased upon removal of MG132. Also, MG132 and 37 degrees C had additive effects in causing trehalose accumulation. Thus, the resistance to heat induced by proteasome inhibitors is not just due to induction of hsps but also requires a short-lived metabolite, probably trehalose, which accumulates when proteolysis is reduced.

Sphingolipid metabolism, apoptosis and resistance to cytotoxic agents: can we interfere?

Sphingolipid metabolism assumes a key role in the complex mechanisms regulating cellular stress responses to environmental stressors, including cytotoxic agents. The sphingolipid metabolic pathways, therefore, are promising sources of anticancer therapeutic strategies. Several sphingolipid metabolites have recently been shown to have bioactivity, and their individual contributions to the regulatory pathways that govern cell growth are currently being established in mammalian cells and yeast. The Sphingomyelin (SM) cycle represents a novel antiproliferative, sphingolipid-mediated signal transduction pathway that regulates cell cycle arrest, differentiation, and apoptosis in response to growth factor deprivation, cytokines, ionizing radiation, heat, and chemotherapy. Ceramide, the putative second messenger of the SM cycle, has been proposed as a molecular sensor of injury and assumes a fundamental role in the cellular stress response. This review will discuss sphingolipid metabolism within the context of the cellular stress response, the contribution of sphingolipids to chemotherapy-mediated apoptosis, and suggest novel sphingolipid-based strategies in the treatment of malignant disease.

Involvement of yeast sphingolipids in the heat stress response of Saccharomyces cerevisiae

A role for sphingolipids in the yeast heat stress response has been suggested by the isolation of suppressors of mutants lacking these lipids, which are unable to grow at elevated temperatures. The current study examines the possible role of sphingolipids in the heat adaptation of yeast cells as monitored by growth and viability studies. The suppressor of long chain base auxotrophy (SLC, strain 7R4) showed a heat-sensitive phenotype that was corrected by transformation with serine palmitoyltransferase. Thus, the deficiency in sphingolipids and not the suppressor mutation was the cause of the heat-sensitive phenotype of the SLC strain 7R4. The ability of sphingolipids to rescue the heat-sensitive phenotype was examined, and two endogenous yeast sphingoid backbones, phytosphingosine and dihydrosphingosine, were found to be most potent in this effect. Next, the effect of heat stress on the levels of the three major classes of sphingolipids was determined. The inositol phosphoceramides showed no change over a 1.5-h time course. However, the four detected species of sphingoid bases increased after 15 min of heat stress from 1.4- to 10.8-fold. The largest increases were seen in two sphingoid bases, C20 phytosphingosine and C20 dihydrosphingosine, which increased 6.4- and 10.8-fold over baseline, respectively. At 60 min of heat stress two species of yeast ceramide increased by 9.2- and 10.6-fold over baseline. The increase seen in the ceramides was partially decreased by Fumonisin B1, a ceramide synthase inhibitor. Therefore, heat stress induces accumulation of sphingoid bases and of ceramides, probably through de novo synthesis. Taken together, these results demonstrate that sphingolipids are involved in the yeast heat stress adaptation.

Molecular biology of trehalose and the trehalases in the yeast Saccharomyces cerevisiae

The present state of knowledge of the role of trehalose and trehalose hydrolysis catalyzed by trehalase (EC 3.2.1.28) in the yeast Saccharomyces cerevisiae is reviewed. Trehalose is believed to function as a storage carbohydrate because its concentration is high during nutrient limitations and in resting cells. It is also believed to function as a stress metabolite because its concentration increases during certain adverse environmental conditions, such as heat and toxic chemicals. The exact way trehalose may perform the stress function is not understood, and conditions exist under which trehalose accumulation and tolerance to certain stress situations cannot be correlated. Three trehalases have been described in S. cerevisiae: 1) the cytosolic neutral trehalase encoded by the NTH1 gene, and regulated by cAMP-dependent phosphorylation process, nutrients, and temperature; 2) the vacuolar acid trehalase encoded by the ATH1 gene, and regulated by nutrients; and 3) a putative trehalase Nth1p encoded by the NTH2 gene (homolog of the NTH1 gene) and regulated by nutrients and temperature. The neutral trehalase is responsible for intracellular hydrolysis of trehalose, in contrast to the acid trehalase, which is responsible for utilization of extracellular trehalose. The role of the putative trehalase Nth2p in trehalose metabolism is not known. The NTH1 and NTH2 genes are required for recovery of cells after heat shock at 50 degrees C, consistent with their heat inducibility and sequence similarity. Other stressors, such as toxic chemicals, also induce the expression of these genes. We therefore propose that the NTH1 and NTH2 genes have stress-related function and the gene products may be called stress proteins. Whether the stress function of the trehalase genes is linked to trehalose is not clear, and possible mechanisms of stress protective function of the trehalases are discussed.

Glucose influence on germ tube production in Candida albicans

The influence of different glucose concentrations was tested in minimal synthetic medium on Candida albicans strain. After 18 hours of starvation, germ-tube (GT) production, amount of consumed glucose, oxygen and the pH of the medium were checked every hour from the beginning through the end of the experiment. Optimal GT production was obtained with 1 g/l of glucose. At this concentration the greatest glucose and oxygen consumption were also noted. No pH variations in the medium were observed in all of the glucose concentrations used. At 3 and 5 g/l glucose concentrations a lower GT production were obtained. The Crab-tree effect might interfere with GT production when glucose concentration is higher than 1 g/l. This data may support the hypothesis that GT production is strictly glucose dependent.

Evidence that the Saccharomyces cerevisiae CIF1 (GGS1/TPS1) gene modulates heat shock response positively

The CIF1 gene (also called GGS1/TPS1) encodes a protein of the trehalose synthase complex that affects trehalose accumulation and general glucose sensing by Saccharomyces cerevisiae cells. There is considerable debate as to whether CIF1-dependent trehalose accumulation is a determinant in heat shock-acquired thermotolerance. Thermosensitivity of cif1 mutants could alternatively, or also, be related to gene expression-signalling defects in such strains. Because many signal-dependent factors are involved in stress protection and repair in yeast, we have compared the expression of various stress response and heat shock genes in 'isogenic' CIF1 and cif1 strains growing exponentially in galactose medium. Transcription of CTT1, CIF1, HSP26, HSP82, HSP104, SSA4 and UB14 was notably lower in the cif1 mutant following heat shock. Moreover, a single copy of chromosomally integrated HSP104-lacZ fusion gave up to 5.5-fold more heat shock induction in the CIF1 strain compared to the cif1 mutant. We conclude that reduced heat shock-acquired thermotolerance in cif1-deletion mutants growing exponentially on galactose is more likely to result from a general reduction in expression of stress response and heat shock genes, than simply or solely through deficiency of trehalose accumulation. The possible role of CIF1 in modulating stress response is discussed.

Increased thermal stability of the enzyme content in permeabilized whole cells from the fission yeast Schizosaccharomyces pombe by exogenous trehalose and other compounds

Cells of the fission yeast Schizosaccharomyces pombe were permeabilized by treatment with toluene-ethanol. The permeabilized cells lost the bulk of the internal trehalose pool while most of the alkaline phosphatase, invertase, alpha-glucosidase, or neutral trehalase activities located inside the cells remained unaffected. This system was used as an in situ assay to determine the involvement of trehalose in enzyme protection during thermal treatments. The addition of trehalose to suspensions of permeabilized cells resulted in a sugar-dependent thermoprotection of the internal marker enzymes. This approach demonstrates that in whole cells of the fission yeast trehalose plays a physiological role as a protective molecule against thermal denaturation of cellular enzymes.

Expression and function of the trehalase genes NTH1 and YBR0106 in Saccharomyces cerevisiae

The biological function of the trehalose-degrading yeast enzyme neutral trehalase consists of the control of the concentration of trehalose, which is assumed to play a role in thermotolerance, in germination of spores, and in other life functions of yeast. Resequencing of the neutral trehalase gene NTH1 on chromosome IV resulted in the observation of two possible start codons (Kopp, M., Nwaka, S., and Holzer, H. (1994) Gene (Amst.) 150, 403-404). We show here that only the most upstream start codon which initiates translation of the longest possible ORF is used for expression of NTH1 in vivo. A gene with 77% identity with NTH1, YBR0106, which was discovered during sequencing of chromosome II (Wolfe, K. H., and Lohan, A. J. E. (1994) Yeast 10, S41-S46), is shown here to be expressed into mRNA. Experiments with a mutant disrupted in the YBR0106 ORF showed, in contrast to a NTH1 deletion mutant, no changes in trehalase activity and in trehalose concentration. However, similar to the NTH1 gene a requirement of the intact YBR0106 gene for thermotolerance is demonstrated in experiments with the respective mutants. This indicates that the products of the likely duplicated YBR0106 gene and the NTH1 gene serve a heat shock protein function. In case of the YBR0106 gene, this is the only phenotypic feature found at present.

Heat-shock response in a yeast tps1 mutant deficient in trehalose synthesis

Exponential cells of the Saccharomyces cerevisiae tps1 mutant underwent a rapid loss of viability upon a non-lethal heat exposure (from 28 to 42 degrees C). However, a further more severe heat stress (52.5 degrees C 5 min) induced an increase in the fraction of viable cells. This mutant can not synthesize trehalose either at 28 degrees C or at 42 degrees C due to the lack of a functional trehalose-6P synthase complex. In control experiments carried out with the wild-type W303-1B, heat-stressed exponential phase cultures grown on YPgal at 28 degrees C acquired thermotolerance to a higher extent than identical cultures grown on YPD, although in both cultures the level of stored trehalose was negligible. These data suggest that the bulk of trehalose accumulated in yeast upon mild heat treatments is not sufficient to account for the acquisition of thermotolerance.

Trehalose inhibits ethanol effects on intact yeast cells and liposomes

The effect of ethanol on stability of intact yeast cells has been investigated. Several strains with differences in trehalose metabolism were examined for their ability to survive in the presence of 10% (v/v) ethanol. A positive correlation was observed between cell viability and trehalose concentration. When leakage of electrolytes from the cells was recorded by observing changes in conductivity of the medium, we found that ethanol increases leakage, but the presence of trehalose reverses that effect. Similar studies were done with liposomes of similar composition to those seen in intact cells in log and stationary phases. In the presence of ethanol, carboxyfluorescein trapped in the liposomes leaked to the medium. When trehalose was added inside, outside or on both sides of the membrane, the ethanol-induced leakage was strongly inhibited. More leakage was observed in liposomes in gel phase state than in liquid-crystalline phase, suggesting that the thermotropic behavior of the lipids in the plasma membrane, together with trehalose, plays a role in enhancing ethanol tolerance.

A family of hexosephosphate mutases in Saccharomyces cerevisiae

The Saccharomyces cerevisiae PGM1 and PGM2 genes encoding two phosphoglucomutase isoenzymes have been isolated and sequenced. The derived protein sequences are closely related to one another and show distinct sequence similarities to the human and rabbit phosphoglucomutases, especially in the region supposed to constitute the active site. PGM1 and PGM2 are located on chromosomes XI and XIII, respectively, just upstream of the known genes YPK1 and YKR2 coding for a pair of closely related putative protein kinases. These observations suggest that an extended region of DNA arose by the process of gene duplication. Cells deleted for both, PGM1 and PGM2, could not grow on galactose. No residual phosphoglucomutase activity could be measured in crude extracts or in permeabilized cells of pgm1/2 double mutants. Unexpectedly, growth with glucose was not impaired and the mutant cells were still able to accumulate trehalose and glycogen, although at a reduced level. Two further genes could be isolated and characterized which when over-expressed on a multi-copy plasmid could restore growth on galactose of the pgm1/2 double deletion mutant. Multi-copy complementation was due to a sharply increased level of phosphoglucomutase activity. Partial sequencing and characterization of the two genes revealed one of them to be SEC53 encoding phosphomannomutase. No extended sequence similarities could be found in the databases for the second gene. However, part of the derived amino acid sequence contained a region of high similarity to the active-site consensus sequence of hexosephosphate mutases from different organism. Further investigations suggest that a complex network of mutases exist in yeast which interact and can partially substitute for each other.

The role of trehalose synthesis for the acquisition of thermotolerance in yeast. II. Physiological concentrations of trehalose increase the thermal stability of proteins in vitro

In baker's yeast (Saccharomyces cerevisiae), accumulation of the non-reducing disaccharide, trehalose, is triggered by stimuli that activate the heat-shock response. Previously, trehalose levels have been shown to be closely correlated with thermotolerance, suggesting a protective function of this substance. Genetic evidence in support of this view is presented in an accompanying paper [De Virgilio, C., Hottiger, T., Dominguez, J., Boller, T. & Wiemken, A. (1993) Eur. J. Biochem. 219, 179-186]. In this study, we have examined the effect of trehalose on the thermal stability of proteins, a parameter thought to be a major determinant of thermotolerance. Physiological concentrations of trehalose (up to 0.5 M) were found to efficiently protect enzymes of yeast (glucose-6P-dehydrogenase, phosphoglucose-isomerase) as well as enzymes of non-yeast origin (bovine glutamic dehydrogenase, EcoRI) against heat inactivation in vitro. Trehalose also reduced the heat-induced formation of protein aggregates. The disaccharide proved to be a compatible solute, as even at very high concentrations (up to 1 M) it did not significantly interfere with the activity of test enzymes. Trehalose was at least as good or better a protein stabilizer than any of a number of other compatible solutes (including sugars, polyalcohols and amino acids), while the structurally related trehalose-6P was devoid of any protective effect. Thermoprotection of enzymes by trehalose was evident even in solutions containing high concentrations of yeast protein or substrate. The data indicate that trehalose accumulation may increase the thermotolerance of yeast by enhancing protein stability in intact cells.

The role of trehalose synthesis for the acquisition of thermotolerance in yeast. I. Genetic evidence that trehalose is a thermoprotectant

In the yeast Saccharomyces cerevisiae, accumulation of the non-reducing disaccharide trehalose is triggered by various stimuli that activate the heat-schock response. Several studies have shown a close correlation between trehalose levels and tolerance to heat stress, suggesting that trehalose may be a protectant which contributes to thermotolerance. In this study, we have examined mutants defective in genes coding for key enzymes involved in trehalose metabolism with respect to the heat-induced and stationary-phase-induced accumulation of trehalose and the acquisition of thermotolerance. Inactivation of either TPS1 or TPS2, encoding subunits of the trehalose-6-phosphate synthase/phosphatase complex, caused an inability to accumulate trehalose upon a mild heat-shock or upon initiation of the stationary phase and significantly reduced the levels of heat-induced and stationary-phase-induced thermotolerance. Deletion of NTH1, the gene coding for the neutral trehalase, resulted in a defect in trehalose mobilization during recovery from a heat shock which was paralleled by an abnormally slow decrease of thermotolerance. Our results provide strong genetic evidence that heat-induced synthesis of trehalose is an important factor for thermotolerance induction. In an accompanying study [Hottiger, T., De Virgilio, C., Hall, M. N., Boller, T. & Wiemken, A. (1993) Eur. J. Biochem. 219, 187-193], we present evidence that the function of heat-induced trehalose accumulation may be to increase the thermal stability of proteins.

Molecular events associated with acquisition of heat tolerance by the yeast Saccharomyces cerevisiae

The heat shock response is an inducible protective system of all living cells. It simultaneously induces both heat shock proteins and an increased capacity for the cell to withstand potentially lethal temperatures (an increased thermotolerance). This has lead to the suspicion that these two phenomena must be inexorably linked. However, analysis of heat shock protein function in Saccharomyces cerevisiae by molecular genetic techniques has revealed only a minority of the heat shock proteins of this organism having appreciable influences on thermotolerance. Instead, physiological perturbations and the accumulation of trehalose with heat stress may be more important in the development of thermotolerance during a preconditioning heat shock. Vegetative S. cerevisiae also acquires thermotolerance through osmotic dehydration, through treatment with certain chemical agents and when, due to nutrient limitation, it arrests growth in the G1 phase of the cell cycle. There is evidence for the activities of the cAMP-dependent protein kinase and plasma membrane ATPase being very important in thermotolerance determination. Also, intracellular water activity and trehalose probably exert a strong influence over thermotolerance through their effects on stabilisation of membranes and intracellular assemblies. Future investigations should address the unresolved issue of whether the different routes to thermotolerance induction cause a common change to the physical state of the intracellular environment, a change that may result in an increased stabilisation of cellular structures through more stable hydrogen bonding and hydrophobic interactions.

Disruption of TPS2, the gene encoding the 100-kDa subunit of the trehalose-6-phosphate synthase/phosphatase complex in Saccharomyces cerevisiae, causes accumulation of trehalose-6-phosphate and loss of trehalose-6-phosphate phosphatase activity

Preparations of the trehalose-6-phosphate synthase/phosphatase complex from Saccharomyces cerevisiae contain three polypeptides with molecular masses 56, 100 and 130 kDa, respectively. Recently, we have cloned the gene for the 56-kDa subunit of this complex (TPS1) and found it to be identical with CIF1, a gene essential for growth on glucose and for the activity of trehalose-6-phosphate synthase. Peptide sequencing of the 100-kDa subunit of the trehalose-6-phosphate synthase/phosphatase complex (TPS2) revealed one sequence to be 100% identical with the deduced amino acid sequence of the upstream region of PPH3 on the right arm of chromosome IV. This sequence was used to clone an upstream region of PPH3 containing an open reading frame of 2685 nucleotides, predicted to encode a polypeptide of 102.8 kDa. The N-terminal sequence, as well as three internal amino acid sequences, obtained from peptide sequencing of the 100-kDa subunit, were identical with specific regions of the deduced amino acid sequence. Thus, the sequence cloned represents TPS2, the gene encoding the 100-kDa subunit of the trehalose-6-phosphate synthase/phosphatase complex. Interestingly, a stretch of about 500 amino acids from the first part of TPS2 was 33% identical with the entire TPS1 sequence. Disruption of TPS2 had no effect on trehalose-6-phosphate synthase activity but caused complete loss of trehalose-6-phosphate phosphatase activity, measured in vitro, and accumulation of excessive amounts of trehalose-6-phosphate instead of trehalose upon heat shock or entrance into stationary phase in vivo. These results suggest that TPS2 codes for the structural gene of the trehalose-6-phosphate phosphatase. Heat shock induced an increase in trehalose-6-phosphate phosphatase activity and this was preceded by an accumulation in TPS2 mRNA, suggesting that the trehalose-6-phosphate phosphatase is subjected to transcriptional control under heat-shock conditions.